Most of the loudspeakers presently used for broadcasting voice or music are electrodynamic loudspeakers. These loudspeakers have wide range of frequency, so they can clearly broadcast various audio signals, but their single-unit acoustic power is limited. Currently, the electrical power of a single loudspeaker is usually under 1,000 Watts. When broadcasting in greater power is needed, it is usually necessary to combine a plurality of loudspeakers; even so, it is still not possible to meet the requirements of large area, long distance or high sound-intensity broadcasting.
A pneumatic loudspeaker uses an electrical signal to modulate a high-pressure airstream, which is similar to the mechanism of human voice generation, and can produce very strong sound. Currently, the acoustic power of a single sound-generating unit can be over 10,000-20,000 acoustic Watts, which equivalent to 50,000-100,000 Watts of the electrical power of an electrodynamic loudspeaker. But the frequency range of a pneumatic loudspeaker is narrow and can hardly be over 2,500 Hz, so its sound definition is bad. The high-frequency (HF) components of sound attenuate more than its low-frequency (LF) components in propagation. At a distant location, as HF components attenuate more, definition is low and it is hard for a listener to hear clearly. Thus, it has difficulty in being used for large-area, long-distance or high sound-intensity broadcasting. And, to effectively modulate an airstream, it is necessary to operate under the state of big amplitude, and the present pneumatic loudspeakers cannot work for a long time without failure; so it is hard for them to be used in audio broadcasting of high sound-intensity.
Ling Electronics developed a loudspeaker that utilized sleeve valve modulation. FIG. 1 illustrates the operation principle of this loudspeaker of Ling Electronics, which has a sleeve valve formed by a movable ring 33 and a still ring 32. Annular slits are provided on the movable ring 33 and the still ring 32, and the slits on the movable ring 33 and the still ring 32 are in alignment with one another respectively. The slits allow the passage of airstream. When the movable ring shifts up and down relative to the still ring, the slits on the movable and still rings will be off-alignment, resulting in change in the cross-sectional area of the airstream passage, which in turn functions as a valve and changes the flow rate of airstream to realize modulation of airstream. The movable ring 33 of this loudspeaker is arranged on the outside of the still ring, so an airstream pushes the movable ring 33 towards the still ring 32; in addition, since the gap between the movable and still rings is very small, the area of contact is big, although the amount of air to be consumed is reduced; thus, a relatively strong frictional force is generated, which tends to result in damage or blockage.
In Chinese Patent No. ZL 92102274.3, the inventor of the present patent application proposed a jet-blocking loudspeaker, which is shown is FIG. 2 in its basic structure. As shown in FIG. 2, the loudspeaker comprises a housing formed by an upper housing body and a lower housing body. The upper end of the upper housing body is an opened port capable of connecting with a horn (not shown); silencer exhaust holes are provided on the side of the upper housing body. An air inlet port is provided at the central portion of the lower end of the lower housing body. A supporting plate is arranged between the upper and lower housing bodies to divide the internal space of the housing into an upper portion and a lower portion, which are communicated with each other through an air channel. A throat canal 2, a central cone 19, an annular nozzle 14, an obstructing ring 15, and a voice coil 13 and its leads are provided within the upper housing body; the lower end of the throat canal forms an annular flow-splitter 20, and the inner side of the flow-splitter and the outer wall of the central cone part form a throat channel. The obstructing ring 15 and the voice coil are pressed into an integral by a lower plate 17 and an upper plate 18 with a rubber support 16, so that the obstructing ring is accurately positioned while it is allowed to displace up and down by the elastic effect of the rubber ring (rubber support). A magnet and an inner magnetic pole 11 and an outer magnetic pole 10 connected with the magnet are arranged in the lower housing body.
Although this loudspeaker overcomes the defects of the sleeve valve pneumatic loudspeaker of Ling Electronics and can be used in high-power sound broadcasting, it has the following shortcomings:
1) The loudspeaker modulates the direction of a jet by obstructing the jet to achieve the object of converting the high-pressure airstream to pressure wave; however, since the annular flow-splitter 20 at the lower end of the throat canal 2 has a blunt-circular shape, the sensitivity of high-frequency modulation is low; in addition, a relatively big gap exists between the obstructing ring 15 and the annular nozzle 14, resulting in significant air leak and low efficiency of airstream utilization. As shown in FIG. 2, at positive half-cycle of signal the obstructing ring 15 moves down to allow the jet to rush into the throat canal 2 to generate acoustic waves, while at negative half-cycle the obstructing ring 15 moves up to direct the jet towards the flow-splitter 20 having a front edge of blunt-circular shape, and as the obstructing ring moves further up the jet is directed by the flow-splitter 20 backward to enter the silencer exhaust hole and is exhausted into the atmosphere. As the obstructing ring 15 is for altering the direction of the jet, there is a relatively big separation between it and the nozzle 14, resulting in that a portion of the airstream leaks through the gap between the obstructing ring 15 and the nozzle 14, and this portion of the airstream has no contribution to the sound-generation and is wastefully dissipated. As the front edge of the flow-splitter 20 used has a blunt circular shape, its flow-splitter sensitivity at high-frequency is low and the high-frequency effect is not satisfactory. Although its air-sound conversion efficiency improved significantly as compared with a sleeve valve loudspeaker, the efficiency of airstream utilization of ZL92102274.3 still needs to be improved.
2) As the obstructing ring 15 is made by titanium, eddy displacement current will be generated in a strong magnetic field, resulting in damping effect and heat generation; the higher the frequency, the greater the damping effect and the heat generation. Thus, the modulation efficiency at high frequency is relatively low, the temperature during operation is very high, and long-time stable operation is difficult to be realized. As the obstructing ring 15 is made by titanium (specific weight: 4.5), it is still relatively heavy, its resonance frequency is still relatively low, and its amplitude at high-frequency is relatively small, and it can hardly realize effective modulation; therefore, its air-sound conversion efficiency at high-frequency is low, its high-frequency range is generally below 3,500 Hz, and its definition at a distant location is inferior.
3) The signal power applied during the operation of the signal coil—voice coil 13 is relatively high (usually in the range of 400-2,000 Watts or higher), resulting in generation of a relatively large amount of heat, and its heat-dissipating measure is not adequate. The temperature of the exhausted gas of a general industrial air-compressor can often reach 90C., plus the heat produced by the eddy current generated in the integral metal voice coil 13 moving in a strong magnetic field, the temperature of the obstructing ring 15 and the voice coil 13 at operation is high and they are easy to fail, so they can not sustain long-time and fault-free operation.
4) Since the amplitude of the obstructing ring 15 is relatively large, the leads of the voice coil 13 are made of knitted copper threads, which is subject to breaking-off due to fatigue, leading to operation failure.